Cameras in vehicles are used both for representation on a display and in driver assistance systems. In this connection, front cameras monitor the road surface region in front of the vehicle from the interior of the vehicle; accordingly, their optical axis determined by the camera lens system and sensor is oriented substantially parallel to the longitudinal axis of the vehicle.
Therefore, the optical path runs from the external coverage region to be imaged, through an optical transit surface of the vehicle window and the camera lens system, to the image sensor. The size of the optical transit surface of the vehicle window is a function of, first of all, the aperture angle of the camera lens system, and secondly, the inclination of the vehicle window with respect to the optical axis; if the window is more inclined or formed to have a flat profile, or the aperture angle is larger, then the optical transit surface becomes correspondingly larger, as well.
However, the available region of the vehicle window required for the optical transit surface is generally limited. In addition to a vehicle camera, further modules may have to be installed, such as a rain sensor, light sensor and, e.g., a mirror support for mounting a rearview mirror in this region. Installed visors, black printing in the upper region of the window for attenuating incident sunlight, and baffles, further reduce the encumbered view through the windshield. Thus, the demand for cameras having a large aperture angle for being able to substantially monitor the scene of the roadway in front of and, if indicated, in the vehicle, conflicts with the limited, available region of the windshield, in particular, in the upper region of the windshield of the vehicle.
In order to be able to provide a large aperture angle of the camera when the optical transit surface is limited, it is suggested, in “Assistenten hinter Glas” [“Assistants Behind Glass”], Automobil-Elektronik [Automotive Electronics], August, 2008, that one or more prisms should be introduced into the optical path between the glass and the camera lens system. The expansion of the optical path may be reduced by such a prism. However, by this means, the reduction in the optical entry surface is limited, based on principle. More intense distortion may occur, which, in the case of the generally discrete-sampling image sensor, may cause significant information losses in some image regions. Furthermore, the optical design of the prism must be adapted to varying window inclination in the respective vehicle, in particular, when a substantially constant line of sight to the front is supposed to be achieved.